With successive reductions in the size of circuit patterns of semiconductor devices in recent years, there is a demand for mask blanks and masks to be used in photolithography conducted under a semiconductor design rule of a DRAM half pitch (hp) of 65 nm or less.
In general, photolithography for microprocessing semiconductor devices is conducted by reduction projection exposure. Thus, the size of a pattern formed in a mask for transferring the pattern is about four times larger than the size of a pattern to be transferred onto a semiconductor substrate. However, the size of circuit patterns transferred onto semiconductor substrates has been reduced to a size considerably smaller than the wavelength of exposure light in photolithography conducted under the semiconductor design rule (DRAM hp of 65 nm or less). For this reason, when reduction projection exposure is conducted through a transfer mask having a transfer pattern that is formed by simply enlarging a circuit pattern by a factor of about 4, the transfer pattern cannot be accurately transferred to a resist film on a semiconductor substrate due to influences such as interference of exposure light.
To deal with this problem, a super-resolution mask is used. One example of such a super-resolution mask is an OPC (Optical Proximity Effect Correction) mask which utilizes an optical proximity effect correction technique and is used to correct an optical proximity effect that degrades the transfer property of the mask. Another example of such a super-resolution mask is a phase-shifting mask (enhancer mask) having a configuration (mask enhancer) in which a phase shifter is provided in the center portion of a light-shielding pattern having the shape of a line or the like. With this configuration, the light-shielding property of the mask pattern is enhanced to increase the resolution of the line pattern. Such an OPC mask requires formation of an OPC pattern, for example, that has a size equal to or less than half that of a circuit pattern (for example, an assist bar, a hammer head, or the like having a linewidth of less than 100 nm). Such an enhancer mask also requires a light-shielding pattern and a phase shifter that have extremely small linewidths.
Such an enhancer mask has the following characteristic. When the width of a light-shielding pattern and the width of a phase shifter are adjusted such that the intensity of light that reaches the backside of the light-shielding pattern from around the light-shielding pattern is balanced with the intensity of light that passes through the phase shifter, the intensity of the amplitude of light that passes through the mask enhancer has a distribution of 0 at a position corresponding to the center of the mask enhancer, and the intensity (amplitude intensity squared) of light that passes through the mask enhancer also has a distribution of 0 at a position corresponding to the center of the mask enhancer.